Foggy windows have two completely different causes that happen to look alike, and they have opposite price tags. One is a humidity problem you can fix this weekend for almost nothing. The other is a failed sealed glass unit. Guessing wrong is how people end up replacing healthy windows — or running a dehumidifier for a year against a problem it can't touch. So start with the test that tells them apart.
Wipe the inside surface. If your finger clears the fog and leaves a streak, the moisture is on the room side of the glass. That's surface condensation — skip to the humidity section below.
Wipe the outside surface. Early-morning fog on the exterior of good windows is common and harmless; it actually means the insulated unit is keeping heat in. If the fog wipes off outside, you're done — nothing is broken.
If neither side wipes clean, it's between the panes. The moisture is sealed inside the double-pane unit where no cloth can reach. That's a failed seal — jump to the section on insulated glass below.
Glass is the coldest surface in the room in winter, so it's where humid indoor air gives up its water first. The air isn't unusually wet in absolute terms — cooking, showers, laundry, houseplants, and plain breathing load a house with moisture, and a tight, well-sealed modern home traps it. When the glass temperature falls below the dew point of the room air, water beads up. Cold snap outside, fog inside: that's the pattern.
The fix is to lower indoor humidity, warm the glass, or both:
If the whole house runs damp year-round, a dehumidifier may be part of the answer; the same habits that work for a damp basement apply upstairs too, which our guide to reducing humidity without a dehumidifier lays out. And if your "fog" turns out to be a window that also sticks and swells in summer, that's moisture getting into the wood itself — our sticky window repair guide covers that side of it.
A double-pane window is a sealed sandwich: two sheets of glass, a spacer around the edge, and dry air or argon in between. The edge seal does the keeping. After years of daily expansion and contraction — glass baking in afternoon sun, then cooling at night — the seal can let go. Humid air creeps in, the desiccant inside the spacer soaks up what it can, and once it's saturated, moisture condenses on the inner faces of the glass where nothing can wipe it away. You may also see a hazy, silvery film or water spots between the panes that never change with the weather indoors.
Your options, in order of how well they actually work:
Live with it. A failed seal is mostly a cosmetic and modest efficiency problem. The fog often comes and goes with temperature and humidity, worse on cold mornings. If the window is in a garage or a spare room, doing nothing is a legitimate choice.
Defogging service. A technician drills tiny holes in the glass, washes and dries the inside, and fits small vents. The window looks clear again — but the insulating gas is gone and not replaced, the unit is now vented rather than sealed, and fog can return. Treat it as a cosmetic procedure with an uncertain lifespan, not a repair of the seal.
Replace the insulated glass unit (IGU). This is the durable fix. A glazier removes just the sealed glass sandwich and sets a new one into your existing frame and sash. The window keeps its frame, trim, and operation; only the glass is new. That's why it typically costs a few hundred dollars per window rather than the price of full replacement — and why a salesperson pushing whole new windows for fog alone deserves skepticism.
| Where the fog is | Meaning | Real fix |
|---|---|---|
| Room-side surface (wipes off inside) | Indoor humidity hitting cold glass | Ventilation, lower humidity, storm windows |
| Outside surface (wipes off outside) | Normal — the unit is insulating well | None needed |
| Between the panes (won't wipe off) | Failed insulated-glass seal | Replace the glass unit (IGU); defogging is cosmetic only |
Replacing whole windows over surface condensation. Brand-new windows will fog exactly the same way if the house air is wet, because the cause was never the window.
Paying for defogging and expecting the insulation back. The clarity may return; the sealed, gas-filled unit doesn't. Get that in writing before you agree.
Ignoring the pattern. One fogged unit is a failed seal. Every window in the house fogging every morning is a humidity problem. The count tells you which fix to buy. If humidity turns out to be your issue and you're shopping for a dehumidifier, our dehumidifier buying checklist covers sizing by room and pint rating.
Wipe first, spend second. Fog you can touch is humidity — chase fans, vents, and indoor humidity levels before you chase window quotes. Fog you can't touch is a failed seal — live with it, or have the glass unit replaced and keep your frames. Whole-window replacement is the answer to rotted frames and broken operation, not to fog. Broken operation usually has a simpler cause, though — a sash that won't stay raised is a balance problem, and our guide to a window that won't stay up covers that fix.
No. It's not a safety or health problem — the glass is intact and structurally fine. The costs are cosmetic (a hazy view) and a modest loss of insulation, since the sealed gas fill has escaped and been replaced by ordinary air.
Some, not all. Even filled with plain air, a double-pane unit insulates far better than a single pane. You lose the extra benefit of the argon fill and the desiccant-dry interior, so the window performs worse than it did — just not catastrophically.
Because that's when the glass gets coldest and indoor air is at its most humid relative to what the glass can handle. The same room air that leaves windows clear in October condenses in January, when the inner glass surface drops below the dew point.
It varies a lot with sun exposure and climate, but seals often last somewhere in the 10-to-20-year range. South- and west-facing units that bake in afternoon sun tend to fail first, because heat cycling is what works the seal loose.
Not in any way that restores it. The sealed unit is manufactured in controlled conditions with dried air or gas and desiccant inside the spacer; there's no home method that rebuilds that. DIY drilling kits copy the defogging process — cosmetic, temporary, and they void any remaining warranty. The genuine repair is a replacement glass unit fitted by a glazier.